158 



CIRCULATORY SYSTEM 



are very tiny in size, 

 them join together. 



Blood flowing through veins passes 

 valves. When these valves are 

 opened due to back pressure, the 

 blood finds other vessels, sometimes 

 smaller, to traverse. 



They get larger and larger as numbers of 

 The largest ones empty into the heart. 

 Their walls are composed of elastic, 

 connective, and some muscular tissue. 

 There might be a back-flow of blood 

 in the veins were it not for the 

 valves that are found along the walls 

 of the veins. These open in the 

 direction of the blood flow w^hich is 

 toward the heart. If blood tends to 

 flow back, they open, and prevent the 

 blood from moving in the opposite direction. These valves may 

 be seen if you let your hand hang down, and shut oft' the blood 

 supply by holding the wrist tightly. The blood will back up 

 against the mouth of the valves and open them. As blood fills 

 these valves, a slight distension of the vein will be noticeable. In 

 some older people, these valves are likely to thicken and show as 

 swellings on the body. Since so small an amount of muscular tissue 

 is present in the walls, very little muscular contraction is possible. 

 The movement of blood in 

 the veins is caused by the 

 heart which is aided CTeatlv 

 by the valves in the veins, 

 by muscles, and by the 

 lungs. The network of 

 capillaries feeding the veins 

 increases the flow just as 

 tributaries to a river cause 

 the flow of water in the 

 river. As breathing takes 

 place, pressure on the large 

 veins in the chest is released. This causes the blood to flow up 

 through the veins. Skeletal muscles in all parts of the body 



The valves in veins are like tiny watch-pockets. If 

 blood tends to flow back, the valves fill up and close 

 the passage; thus blood is kept moving in one direction. 



